1. Field of the Invention
This invention relates generally to image sensors, and more particularly to failure detection in image sensors.
2. Description of the Background Art
Electronic image sensors are commonly incorporated into a variety of devices including, for example, cell phones, computers, digital cameras, PDA's, etc. In addition to conventional user-controlled still and video camera applications, more and more image sensor applications are emerging. For example, integral machine vision applications are expanding rapidly in the automotive, manufacturing, medical, security, and defense industries. In such applications, machines typically perform certain operational tasks (e.g. collision prevention tasks) based on information (e.g. position of an object relative another object) captured by the image capture system of the machine. In order for the machine to perform the proper task associated with the particular situation, it is essential for the image sensor to reliably capture, process, and output image data that accurately represents the observed situation.
A Complementary Metal Oxide Semiconductor (CMOS) image sensor typically includes a sensor array, control circuitry, row control circuitry (e.g., row address decoder, pixel drivers, etc.), column sampling circuitry, and image processing circuitry. Image sensors are often used in conjunction with a lens assembly which is aligned with the sensor array so as to focus an image thereon. The sensor array converts incident light into electrical data indicative of the image. The sensor array is made up of a plurality of light sensitive pixels arranged in a plurality of rows and columns. The pixels are electrically coupled to the row control circuitry and the column sampling circuitry via a grid of row and column signal lines, respectively. That is, each individual row of pixels is connected to, and controlled by, the row control circuitry via an associated set of row signal lines including, for example, a transfer line, a reset line, and a row select line. Each individual column of pixels is connected to the column sampling circuitry via a discrete column sampling line. The column sampling circuitry typically includes sampling components such as, for example, amplifiers, analog-to-digital converters, and data storage elements that are coupled to the column sampling lines for digitizing and storing the electrical signals output from the pixels. In image sensors that have a column parallel readout architecture, the column sampling circuitry includes a discrete set of these sampling components for each column sampling line such that an entire row of pixels can be sampled simultaneously. In column-parallel readout architectures, the column sampling circuitry also includes various signal lines that are routed to the various sampling components so as to carry control signals thereto. (Non-column parallel readout architectures also require various horizontal signal lines, although not as many as a column parallel architecture.) The image processing circuitry receives digitized data output from the column sampling circuitry and generates image data in readable format. The interface enables the image sensor to communicate (e.g., output formatted image/video data, receive operating instructions, etc.) with a host system (e.g., cell phone motherboard, vehicle computer system, manufacturing machine computer system, etc.). In general, the control circuitry of the image sensor is connected to the row control circuitry, the column sampling circuitry, the image processing circuitry, and the interface so as to carry out various timing and control operations.
Each pixel includes a photosensitive element (e.g., photodiode, photogate, etc.), a transfer transistor, a floating diffusion region, a reset transistor, a source-follower transistor, and a row-select transistor. The photosensitive element is operative to accumulate a charge proportional to the intensity of incident light to which it is exposed during shutter operations. The transfer transistor connects the photosensitive element to the floating diffusion region and includes a gate that is connected to and, therefore, controlled by a single transfer line dedicated to the entire row of pixels. When a logical high voltage signal is asserted on the transfer line, the charge from the photosensitive element is transferred to the floating diffusion region. The reset transistor connects the floating diffusion region to a voltage source terminal and includes a gate that is connected to and, therefore, controlled by a reset line of the row signal lines. When a logical high voltage signal is asserted on the reset line, the reset transistor connects the floating diffusion region to the voltage source terminal, thus resetting any previously stored charge to a known state. The source-follower transistor connects the voltage source terminal to the row-select transistor and includes a gate that is connected to the floating diffusion region so as to generate an amplified voltage signal indicative of the charge accumulated within the floating diffusion region. The row-select transistor connects the source-follower transistor to the pixel output line of the column lines and includes a gate that is connected to a row-select line of the row lines. When a logical low voltage is asserted on the row-select line the row-select transistor acts as an open switch between the source-follower transistor and the pixel output line. Oppositely, a logical high voltage asserted on the gate of the row-select line causes the row-select transistor to act as a closed switch between the source-follower transistor and the column sampling line such that the state of the floating diffusion can be sampled through the column sampling line.
Although traditional image sensors meet the needs of many image and video capture applications, there are drawbacks to current designs. For example, CMOS pixels are constructed from integrated circuit components (e.g., transistors, diodes, capacitors, etc.) that are prone to failure. As another example, pixel row signal lines (e.g., transfer lines, reset lines, row-select lines, etc.), column sampling lines, and column sampling component control lines (e.g., gain amplifier control lines, analog-to-digital converter control lines, digitized pixel data storage device control lines, etc.) are prone to damage, especially those subjected to large distributed stress-causing loads. As yet another problem, row control circuits are also prone to failure. In the event that any of the aforementioned failures occur in a conventional image sensor, it will generally output erroneous image data to the hosting system. Of course, a hosting system typically does not recognize the difference between erroneous image data and correct image data. This can be particularly problematic in certain applications (i.e. integral machine vision applications) wherein the image data dictates operational tasks performed by the host system. Even when the circuits are not very prone to damage or failure, certain applications (e.g., automotive applications) demand systems with exceptionally high reliability.
What is needed, therefore, is an image sensor design with improved image data output reliability.